1. Field of the Invention
The present invention relates to a differential type gate logic circuit and, more particularly, to a differential type gate circuit which is suitable to select and output an input signal of a differential type gate logic circuit.
2. Description of the Prior Art
In a logic gate circuit having a fixed threshold value, when an input voltage exceeds this threshold value, turn-off and turn-on of transistors as component elements of the gate circuit start and the output voltage changes.
In such a logic gate circuit, when a noise is added at the rise time of the input voltage, the input voltage will have exceeded the threshold value early, so that the output voltage also changes early. Such fluctuation in the output voltage occurs in the logic gate circuit of the fixed threshold type.
A differential type gate circuit is used to solve such a problem. In the differential type logic gates, each input signal and its inverted signal is given as one set (This is called complementary input signals.). With respect to each input signal, the output voltage starts changing when the voltage of the input signal equals to the voltage of its inverted signal.
Even when a noise occurs at the rise of the input signal, both the input signal and its inverted signal are influenced. Thus, the time point when the voltage of the input signal equals the voltage of the inverted signal does not change. Therefore, the early change the output voltage due to noise is prevented.
However, unlike the gate circuit having a fixed threshold value, the differential type gate circuit has a limitation such that the input signal must always be of the complementary type. Therefore, it is impossible to further apply a control signal as an input signal to the gate circuit. Consequently, in the case where the control signal is needed, it is necessary to constitute the differential type gate circuits in multiple stages. This means that an extremely large amount of hardware is required as compared with the fixed threshold gate circuit.